The present invention relates to a method of manufacturing a golf ball in which a material molded under heat from a rubber composition serves as a ball component. More specifically, the invention relates to a golf ball manufacturing method which includes the step of preparing a masterbatch of an unsaturated carboxylic acid and/or a metal salt thereof.
Many golf balls that use rubber compositions containing polybutadiene polymerized with a rare-earth catalyst have hitherto been described in the art. Such golf balls axe disclosed in, for example, U.S. Pat. Nos. 6,695,716, 6,712,715, 6,786,836, 6,921,345, 6,634,961 and 6,602,941 (Patent Documents 1 to 6). However, there remains room for further improvement in the rebound performance of such golf balls. Moreover, sufficient performance has yet to be achieved as well in terms of manufacturability.
U.S. Pat. No. 6,642,314 (Patent Document 7) describes the use of an alkoxysilyl group-bearing compound-modified polybutadiene as a rubber composition for golf balls. JP-A 2007-222196 (Patent Document 8) discloses a polybutadiene obtained by additionally subjecting the modified polybutadiene of Patent Document 7 to a condensation reaction. However, in all of the above-mentioned prior art, there remains room for improvement in manufacturability and in the durability and rebound of the resulting golf balls.    Patent Document 1: U.S. Pat. No. 6,695,716    Patent Document 2: U.S. Pat. No. 6,712,715    Patent Document 3: U.S. Pat. No. 6,786,836    Patent Document 4: U.S. Pat. No. 6,921,345    Patent Document 5: U.S. Pat. No. 6,634,961    Patent Document 6: U.S. Pat. No. 6,602,941    Patent Document 7: U.S. Pat. No. 6,642,314    Patent Document 8: JP-A 2007-222196
Moreover, it is regarded as desirable for the durability and rebound of a golf ball to use, as the metal salt of an unsaturated carboxylic acid typically employed as the co-crosslinking agent in a rubber composition for golf balls, a compound such as zinc methacrylate or zinc acrylate. This zinc methacrylate or zinc acrylate is generally included in a large amount with respect to rubber ingredients such as polybutadiene. However, when the various ingredients are masticated to form a rubber composition, because the zinc methacrylate and zinc acrylate are in the form of a fine powder, a considerable amount of the powder scatters or sticks to the roll mill or other kneading apparatus, greatly interfering with the masticating operation.
Moreover, because these compounds readily form aggregates within the composition and have a poor dispersibility, the unsaturated carboxylic acid zinc salt included in the composition is not effectively utilized, which may lower the resilience and make a constant hardness impossible to achieve.
One commonly used method for preventing the metal salts of unsaturated carboxylic acids from sticking to the kneading apparatus is to employ the metal salt of a higher fatty acid as a lubricant. However, such compounds are ineffective unless the mastication temperature is set to a high temperature of about 100° C. Unfortunately, scorching occurs at high temperatures.
In JP 3178857 (Patent Document 9), an α,β-ethylenically unsaturated carboxylic acid metal salt suspended in an aliphatic hydrocarbon solvent is mixed with a polymer solution to form a rubber composition. However, in this method, following liquid mixture, the solvent must be removed.
JP 2720541 (Patent Document 10) describes the use of a rubber composition prepared by first mixing and dispersing the metal salt of an unsaturated carboxylic acid in liquid rubber, followed by mastication. However, the liquid rubber used is thought to lower the rebound of the golf balls obtained.
To resolve the above problems, JP-A 2004-105680 (Patent Document 11) discloses art in which a metal salt of an unsaturated carboxylic acid is incorporated as a co-crosslinking agent into a rubber composition after first being mixed with and dispersed in a solid rubber such as 1,4-polybutadiene.
However, in the molded and vulcanized materials obtained by the foregoing art, there remains substantial room for improvement in resilience and durability.    Patent Document 9: JP 3178857    Patent Document 10: JP 2720541    Patent Document 11: JP-A 2004-105680